Progesterone markedly inhibitis the functions of human macrophages and T lymphocytes, and acts as an immunosuppressant during pregnancy. It is important to examine progesterone metabolites to understand the overall bioactive properties of this sex steroid. However, progesterone metabolism has not been examined in human immune cells. The human leukemic monoblast U937 cell line exhibits monocytic lineage and provides a valuable model to analyze monocyte-macrophage differentiation. Therefore, in this study, we analyzed progesterone metabolism in U937 cells by thin-layer chromatography. Progesterone was metabolized to 5α-pregnan-3β,6α-diol-20-one
via 5α-dihydroprogesterone and 5α-pregnan-3β-ol-20-one, and 5α-pregnan-3β,20α-diol was also detected as a final metabolic product
via 20α-dihydroprogesterone and 5α-pregnan-20α-ol-3-one. 5α-reduction (5α-reductase type 1) and 20α-reduction were involved in the first step of metabolism. To identify the enzyme responsible for the 20α-reduction, we screened an U937 cDNA library, and obtained a clone (1.2 kb), which was identical to the human hepatic bile acid-binding protein or 20α-hydroxysteroid dehydrogenase (20α-HSD). 293 cells transfected with this cDNA demonstrated marked 20α-reduction of progesterone to 20αDHP, but 20α-oxidative, 3α-HSD or 17β-HSD activity was found to be negligible. In experimental animals, the importance of 20α-HSD has been reported to be involved in the protection of immune cells from the toxic effects of progesterone. Therefore, our present data suggest that 20α-HSD plays an important role in the reguation of progesterone actions in human immune cells.
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